You probably will not see one of those because it would vary with the software. Some software is CPU intensive. Some is GPU intensive. So the break even point would vary.
I do not worry about it. I tend to bias things toward a more powerful CPU. I have a Q9550 OC'd to 3.6 GHz. That has been good enough to let me skip the last two upgrade cycles and let me wait for Ivy Bridge.
An i-K running at 5.0 GHz is a pretty powerful processor. It would take a lot to bottleneck it.